PROCEDIMIENTO DE CONTRATACIÓN A) Justificación de la necesidad de las

The following discussion extends the previous concrete and asphalt com- parison example, and considers further pavement application options in Western Australia.*

Recycled concrete, recycled demolition-arisings and recycled asphalt materials are compared with raw material equivalents at three life cycle periods of up to 90 years to assess environmental/economic considerations for local pavements, namely,

• Footpath alternatives compared over a lifetime toward user benefit • Four alternative footpath specifications were considered: (i) asphalt,

(ii) in situ concrete, (iii) brick paving and (iv) limestone paving options. • Paths were designed to the same loading and performance param- eters for a new housing development requiring 1000 m of a 1-m-wide path.

• Road base recycled materials were compared against virgin raw materials. • Structurally fit-for-use recycled concrete road base (CCRB) versus

recycled demolition road base (CDRB) materials were matched against virgin limestone.

• Road wearing course recycled asphalt pavement benefits, mainte- nance and whole-life costs were considered.

• Three methods are in use locally for heavily trafficked roads: (i) standard hot mix asphalt, (ii) warm mix asphalt and (iii) warm mix asphalt containing recycled asphalt pavement; considering environmental and social merits or otherwise, these options are analysed over road life span.

A discount rate, this time calculated at just over 6% and testing over various life spans, finds that sustainable (recycled) options are superior to (virgin) specifications when considering the range of alternative pavement specification design options across the triple bottom-line criteria of eco- nomic, environmental and social factors. An Excel spreadsheet framework was programmed and all cost information was entered, formatted and interpreted. Once values had been ascertained, they were entered. Data plots were then made and alternatives compared for best life cycle benefit (Table 2.14).

Four footpath options (1000 m × 1 m, 1000 m3_{) were considered for 60}

(and 30) years at a discount rate of 6.25% (11% and 3%):

• Asphalt footpath: 100-mm-thick cement-treated crushed rock base course with 25 mm bituminous surface

• Concrete footpath: 100-mm-thick unreinforced 32 Mpa concrete • Brick footpath: 75-mm-thick, 110 mm × 230 mm standard clay

brick

• Limestone footpath: 75-mm-thick rectangular stone

In this case, although concrete has a higher initial cost compared with asphalt, it is the cheapest option over a life cycle.

After 10 years, a concrete pavement, if chosen, will save the taxpayer money despite the cheaper initial cost of asphalt.

Concrete ends up being $7337 cheaper over 60 years; a reasonable saving. The other two options’ initial costs are so high that no amount of life span saving is likely to attract clients or their design representatives.

The limestone paving stone option scored best (after review of user prefer- ences for aesthetics, surface comfort and enhanced property value perception);

Table 2.14 Footpaths Item cost

Footpath pavement options ($/1000 m3₎

Asphalt Concrete Brick Limestone

Capital 28,650 33,330 59,400 105,000 Replace 17,858 5715 9784 17,181 Maintenance 21,905 21,716 22,315 21,905 Salvage 61 407 −254 77 Life cycle 68,475 61,138 91,245 144,164 Benefit 70 103 93 118 B/C 1.0 1.7 1.0 0.8 Rank 2 1 2 4

however, it’s restrictively high (doubled) cost excludes it from any real practi- cable consideration.

Brick pavers scored well, with an overall benefit–cost (B/C) ratio of 1.0. Over the pavement life cycle, brick (although $22,770 more expensive than the bituminous option) makes gains against the B/C score of (the users apparent dislike of) asphalt.

Overall, for footpaths, with a confidence index of 1.53 (in which 0.25+ is considered positive), the concrete option is deemed the most practicable pavement option here.

The road base case study concentrates on initial and salvage costs (main- tenance road base costs are nominal) and haulage. Three road base options are available:

• Limestone virgin material • Recycled concrete (grade 1)

• Recycled demolition material (grade 2)

These are assessed over the life of a heavily trafficked road (a bus lane construction of 1000 m3_{), for a practicable period of 51 years (Table 2.15).}

• New limestone is initially cheaper than the recycled options (assum- ing that haulage distances are equitable).

• The two recycled options gain superior nonmonetary benefits. Grade 2 is more workable so it receives a slightly higher score than grade 1. • Considering the B/C ratio, virgin limestone would become a more

attractive option (compared with the recycled grade 2 alternative). • Currently, Western Australia’s recycling facilities are much less abun-

dant than quarry facilities.

Table 2.15 Road base options

Item cost

Road base options Virgin

limestone Recycled concrete (Grade 1) Recycled demolitions (Grade 2)

Capital 9783 15,600 12,000 Replace 7667 10,725 11,000 Maintenance 13,423 15,000 13,500 Salvage 1931 854 876 Life cycle 32,783 42,179 37,376 Benefit 115 141 149 B/C 3.5 3.3 4.0 Rank 2 3 1

Design engineers involved in road base specification decision making might be justified in a choice of virgin limestone in this scenario.

Wearing course specification options include

• Hot mix asphalt, which is a traditional method using high tempera- tures (160 degrees) to achieve maximum workability and bonding. • Warm mix asphalt, which is a newer and increasingly popular inno-

vation that lays ‘warm’ (100 degrees) asphalt.

• These two methods can be combined with a third sustainable option of using recycled asphalt pavement with virgin materials.

These three road wearing course options are deemed most practicable for 250 m2 _{material coverage (at a depth of up to 400 mm; Table 2.16).}

• The options are very close in terms of cost and social/environmental virtues.

• Initially, the more environmentally friendly options are slightly cheaper.

• Nonmonetary analysis scores benefit the two sustainable options, although hot mix asphalt has better constructability and durability. In general, the wearing course option using warm mix asphalt with recy- cled asphalt pavement is deemed the best option for this scenario.

Overall, it might be suggested that in this case study

• In situ concrete should be used for the construction of residential

footpaths.

• Recycled road base materials should be used, where haulage does not preclude application.

Table 2.16 Wearing course options

Cost Hot-mix asphalt Warm-mix asphalt Recycled asphalt

Capital 40,876 39,658 38,242 Replace 8713 11,008 10,615 Maintenance 37,286 39,150 40,574 Life cycle 86,875 89,816 89,431 Benefit 94 98 99 B/C 1.08 1.09 1.10 Rank 2 3 1

• Warm mix asphalt, which incorporates recycled asphalt pavement (at a proportion of 10%), should be used for the wearing course of Western Australia’s highly trafficked arterial roads.

• LCCA techniques should be integrated into design decision making in Western Australian construction and civil engineering projects towards the prediction of potential whole-life cost savings